The Perfect DC/DC For IoT Sensor Nodes

   

For once Battery technology and electronics are aligning pretty well for portable equipment. Sure in the last Century we could use some sort of AA Battery holder and get our supply voltage in 1.5 Volt increments, but with today's single Lithium cells operating over a 3 to 4.2 volt range and our 32 Bit Processors able to operate down to 2.5 volts, it is a far simpler task to design a small, battery-operated smart sensor node.

Sensor Operation

Sensors are naturally the heart of any remote sensing scheme. They typically run on 3 to 5 volt power supplies, and in the case of air quality sensors can draw anywhere from 30 mA to 100 mA. If your sensor node is wireless, the Radios also can draw upwards of 50 mA when transmitting.

While 100 mA at 5 Volts is a lot of power, the good news in most of these remote sensors is that they do not need to stream a lot of data. In the case of a recent Air Quality sensor project I did, it didn’t need to read faster than about 1 reading every 10 minutes, since the air quality really can’t change all that quickly. The Wireless data link and processor on time were similarly limited to just a few seconds of operation every 10 minutes, making for a very favorable power profile.

This operational profile leaves a lot of potential power optimization potential for extending battery life or reducing battery size.

Simple Sensor DC/DC

The Air Quality sensor that I was working on required two 5 Volt Sensors, an Air Particle sensor, and a CO2 sensor, the rest of the circuitry was designed to run on a continuous 2.9 Volt low power, linear regulated supply.

What is needed then to power the sensors is a 3 to 4.1 input voltage range and a 5 volt output at up to 100 mA, a further requirement of an enable control was needed to turn the sensors on only for a reading and a power good signal was required also so that the processor can judge and transmit the condition of the measuring system.

Looking around at the various manufacturer's section guides turned up the LTC1751-5, an Inductorless Switch Capacitor Voltage Doubler.

Inductorless is nice to remove the radiated EMI of a boot converters inductor from being a possible problem and possibly reduce the overall size of the DC/DC.

Figure 1 shows the basic circuit configuration of the converter. As mentioned the Power Good signal is useful for the Microprocessor to monitor if the power is good, this is an early warning that something is wrong with the sensor and this information can be sent with the wireless data back to the sensor data collection hub.

  

Figure 1 – The application circuit of the LTC1751-5 is the definition of simplicity itself. No inductors and even a Power Good output.

For instance, if the Microprocessor reads an out-of-bounds sensor value – what does that mean? But if the Microprocessor reads a out-of-bounds sensor value AND the power good signal is low, that is almost certainly a shorted sensor and the Microprocessor can not only tell the data collection node that the sensor is faulty, but the Microprocessor can also turn that sensors power off to make sure that there isn’t excessive power drain.

Application Hints

The circuit of figure 1 is nice and compact, but there are several points that are worth noting,

#1 - The output voltage ripple can be quite large – the LTC1750-5 datasheet shows 100 milli-volt peak to peak at 100 kHz with the circuit values shown. To get these values you must take care with the capacitor selection. This is one place where a ceramic capacitor's voltage coefficient can cause you trouble. Carefully look at the capacitors data sheets to make sure that your selected 10 uF output capacitor isn’t acting like a 1 uF capacitor with 5 volts of DC bias [2]. The same caution applies to C1 and C4. A couple of eye-opening articles on this subject are noted in Reference [2] below.

Capacitors of the rated value with DC Bias with low ESR are a must, and possibly secondary filtering may be required. To keep the noise levels low. See Figure 2.

#2 – The input current ripple from the LTC1751-5 running at full load is in excess of 100 mA peak to peak at 100 kHz. If operating on a small battery be sure that this amount of ripple won’t cause undue stress on the battery, especially its protection circuitry. Extra filtering may be required. A small, low ESR Super Cap may be an optimum solution to buffering the battery. Less optimum is the addition of a small LC input filter, but it can do the job very well.

#3 – Inrush, or start-up current may be a factor when running on a small battery, be sure to take this into consideration when designing the circuit. A large inrush current may cause the battery's short circuit protection to activate, especially at low temperatures.

Figure 2 – Three ceramic capacitors rated at 6.3 Volts were measured on my bench versus DC bias. The only totally safe one is the upper trace, a COG type (Blue curve above). COG types are the largest of ceramic capacitor family, but also always have a very low capacitance change with DC bias. Even X7R types can be worse than you think! Check every capacitor datasheet carefully [2].

#4 – Output noise is potentially considerable even in the suggested circuit on the datasheet, the output noise is in the 50 to 75 millivolt peak to peak range. Again you may need to increase the output capacitance and or C4 or use secondary filtering. See the datasheet for some suggestions.

#5 – Turn-on time – you have a few adjustments for turn-on time. Larger output capacitors will slow turn-on time as will adjusting the “Soft Start” capacitor (C3 in Figure 1).

#6 – Simulate or be prepared to rework: Of course, nothing beats an actual breadboard of a circuit for testing, but Linear Technology provides a “Test Jig” circuit of the LTC1751-5 for use in LTspice, it seems to simulate the noise and turn-on characteristics accurately.

#7 – If you do use an inductor-based LC filter to reduce the conducted noise, do yourself a favor and ALWAYS use a ‘Shielded’ type construction unless you want radiated noise everywhere [x].
 

Figure 3 – Linear Technology provides a LTC1751 “Test Jig” circuit for use in their LTspice circuit simulator, it is highly recommended that you simulate your circuit before committing to an actual PCB. Do be sure to accurately model the capacitor ESR - because this default Test Jig From Linear Technology uses ideal capacitor models, that's kind of overly optimistic for any power supply! Power Supply Circuits Live and Die based on the capacitors ESR!

Alternatives

You might be thinking: “Why not just use a regular Boost DC/DC Converter, it has a built-in LC filter on the input that reduces input current ripple, so it might be simpler in the long run.” See Figure 4.

 

Figure 4 – A standard Boost DC/DC converter has the power inductor on the input circuit that reduces the input ripple current, and that can simplify the overall application circuit. But it does have a potentially major fault for a battery-operated circuit, and that is it has no inherent short circuit protection. If the output is shorted for whatever reason, the controller can do nothing abort it as there is a straight-through path from the input to the output through the inductor to the boost diode to the short circuit.

And that is a valid point, but in my battery application, the short circuit protection was a requirement to keep the main application running in case of an Air Quality Sensor failure. That short circuit protection combined with the “Power Good” signal out of the LTC1751-5 gave me a level of onboard diagnostics to be able to deal with a potential sensor failure.

Bonus Information

Some manufacturers have wonderful online/interactive tools that allow you to pick any of their ceramic capacitors and get a display of all the pertinent parameters including ESR and the effect of DC Bas on the capacitance value.

One such tool is from the AVX Website [4], and picking a typical 10uF, 6.3V, X7R, 0805 capacitor produces this result,

 

Figure B1 – A typical Capacitance value drop of a 10 uF, 6.3V, X7R 0805 capacitor that took only seconds to generate from the AVX website.

 

Figure B2 – A typical Capacitance value drop of a 10 uF, 6.3V, X7R 0805 capacitor that took only seconds to generate from the Murata website.

Beware #1 – This is typical data only – i.e. Even the manufacturer didn’t go measure each, and every one of their thousands of different capacitors.

Beware #2 – Don’t think that all manufacturer's capacitors will perform the same as these will, while they might be similar, they won’t be the same and you could still expect to see variation from these capacitors from some other brand. If there isn’t an interactive tool, then you will just have to search the datasheets, or better yet, make your own measurements.

References

[1] Linear Technology LTC1751 Data Sheet

[2] Mark Fortunato, Maximum Integrated Circuits Tutorial, TU5527,
https://pdfserv.maximintegrated.com/en/an/TUT5527.pdf

Also this excellent article,
https://passive-components.eu/dc-and-ac-bias-dependence-of-mlcc-capacitors-including-temperature-dependence/

[3] Hageman, "Friends don't let friends use un-shielded inductors",

https://analoghome.blogspot.com/2017/10/friends-dont-let-friends-use-un_5.html

[4] I am sure that these links will be broken in just a few months from now, but it will be on the AVX and Murata websites somewhere, just go there and search for them....
 

AVX Online Selector Tool,

https://spicat.kyocera-avx.com/mlcc
 

Murata online tool, they call it ‘SimSurfing’
https://ds.murata.co.jp/simsurfing/mlcc.html?lcid=en-us

These sorts of tools from manufacturers are always changing - be sure to check all the manufacturers websites for the latest available information. 

Article By: Steve Hageman www.AnalogHome.com    

We design custom: Analog, RF and Embedded systems for a wide variety of industrial and commercial clients. Please feel free to contact us if we can help on your next project. 

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